This invention relates to a control device for a brushless motor and a control method therefor, and more particularly to a control device for a brushless motor adapted to excite driving coils of a stator in turn depending on an output of a position detector for detecting a position of a rotor, to thereby rotate the rotor and a control method therefor.
A brushless motor generally includes a rotor having a magnet field and a stator including driving coils for multiple phases. A control device for the brushless motor is constituted by a position detector for detecting a position of magnetic poles of the rotor and a motor driving section for changing over an excitation phase of each of the driving coils of the stator depending on an output of the position detector (determining driving coils to be excited to flow an exciting current to the driving coils determined in turn).
Referring now to FIG. 10, an essential part of a brushless motor of the three-phase outer rotor type is illustrated. In FIG. 10, reference numeral 1 designates a rotor and 2 is a stator. The rotor 1 includes a yoke 100 formed into a cup-like shape and a magnet field 101 mounted on an inner periphery of the yoke 100. The magnet field 101 may comprise a ring-like magnet having two magnetic poles which is mounted on the inner periphery of the yoke 100. The stator 2 includes an armature core including an annular yoke section 201a and three silent-pole sections 201u to 201w radially projected at angular intervals of 120 degrees from the yoke section 201a, as well as driving coils 202u to 202w for three phases U, V and W wound on the silent-pole sections 201u to 201w of the core 201, respectively.
3u to 3w are position sensors for the three phases U, V and W which constitute a position detector. The position sensors 3u, 3V and 3 W are arranged in correspondence to central positions of the silent-pole sections for the phases V, W and U, respectively. The position sensors 3u to 3w function to detect a polarity (N pole or S pole) of the magnet field of the rotor 1 opposite to each of the silent-pole sections, to thereby detect a position of the rotor 1 with respect to each of the driving coils 202u to 202w for the three phases. In other words, the position sensors detect positional relationships between the driving coils for the three phases and a plurality of magnetic poles.
FIG. 11 shows an example of a conventional control device for the brushless motor, wherein reference numeral 4 indicates a motor driving section which has three position detection signals input thereto from the position detector 3 comprising the position sensors 3u to 3w and functions to change over an excitation phase of the stator 2 depending on a variation in level of an output of each of the position sensors 3u to 3w. 5 is a rotational speed detector for detecting a rotational speed of the rotor based on a frequency of a signal generated from the position detector 3. 6 is an output control section for controlling the motor driving section 4, which functions to keep the rotational speed of the rotor at an indicated value based on an output of the rotational speed detector 5 input thereto, change over a direction of rotation of the rotor when it is externally fed with a rotational direction changing-over command, short-circuit the driving coils when it is fed with a stop command, or reverse a direction of an excitation current to permit the motor to act as a generator, to thereby brake the rotor.
U.S. Pat. No. 4,054,821 discloses braking of a rotor while using a motor as a generator. U.S. Pat. No. 4,054,819 discloses techniques for reversing an excitation current to brake a rotor during normal rotation of the rotor. Techniques of reversing a direction of flowing of an excitation current to brake the rotor are likewise disclosed in U.S. Pat. No. 3,958,163 and U.S. Pat. No. 3,995,204.
In the brushless motor shown in FIG. 10, when it is required to stop the rotor 1 of the motor at a predetermined position, the position sensors 3u to 3w are arranged so as to permit a predetermined positional relationship to be established between a position at which the rotor of the motor is stopped and a position of each of the position sensors 3u to 3w. Then, the rotational speed of the rotor is gradually reduced while detecting a rotational angular position of the rotor depending on an output of each of the position sensors 3u to 3w, resulting in the rotor being finally stopped at a target position. Alternatively, the above-described braking techniques are utilized to brake the rotor, so that the rotor may be ultimately stopped at the target position.
In the conventional brushless motor, it is required to carry out speed control in a complicated manner in order to stop the motor at a target position, resulting in electronic circuits for the output control section, the motor driving section and the like being complicated. Also, in the brushless motor, various kinds of control is carried out using a microcomputer. However, also in this instance, controlling of the stop position causes a software for operating a microcomputer to be complicated.